The present invention relates to a power switch circuit that switches the power supply for a device between a main power supply and a backup power supply.
An electronic device generally includes a main power supply, which supplies power voltage to a load (internal circuit), and a backup power supply, which is for maintaining the operation of a memory or the like to backup data. The backup power supply functions not only when the main power supply is not used but also when the voltage of the main power supply becomes low to supply the load with operational voltage and maintain the function of the electronic device. Normally, the maximum supply capacity (maximum voltage value) of a backup power supply is set to be lower than the maximum supply capacity (maximum voltage value) of a main power supply.
Japanese Laid-Open Patent Publication No. 6-12876 describes a prior art example of a power switch circuit. The power switch circuit illustrated in FIG. 6 of the publication includes a first transistor, which is formed by a MOS transistor and connected between a main power supply and an output terminal, and a second transistor, which is also formed by a MOS transistor and connected between a backup power supply and the output terminal. The power switch circuit activates the first transistor and deactivates the second transistor to output the voltage of the main power supply to the output terminal. Further, the power switch circuit deactivates the first transistor and activates the second transistor to output the voltage of the backup power supply to the output terminal. This prior art structure, however, has a shortcoming in that current leaks out of the backup power supply if the voltage of the main power supply becomes lower than the voltage of the backup power supply when the main power supply is selected. Under such a situation, the potential at the output terminal becomes lower than the voltage of the backup power supply due to the active first transistor. The leakage occurs as current flows from the backup power supply to the output terminal via a body diode formed in the second transistor. When current leaks out of the backup power supply, the voltage required for operations using the backup power supply, such as data backup, becomes insufficient. This may result in the loss of data. Such a decrease in the supply capacity of the backup power supply may significantly affect the operation of a device.
Japanese Laid-Open Patent Publication No. 2008-86100 discusses a solution to such a current leakage problem. In the publication, two MOS transistors are connected in series between a backup power supply and an output terminal. Further, the back gates of the MOS transistors are connected to each other so that the forward bias direction of each body diode is reversed from that of the other body diode (refer to FIG. 2 of the publication). To facilitate the description hereafter, the connection structure of the two MOS transistors is referred to as a back-to-back (BTB) connection structure. Employment of the BTB connection structure prevents leakage current from flowing from the backup power supply via a BTB connection element even if the main power supply voltage (i.e., potential at the output terminal) becomes lower than the backup power supply voltage. However, as would be understood by one skilled in the art, the BTB connection structure not only causes complications when mounted on an IC chip but also occupies a relatively large layout area in the limited chip region. Accordingly, the BTB connection structure is at least not preferable from the viewpoint of IC mounting.